1. Field of the Invention
The invention relates to an apparatus for the administration of a respiratory gas and at least one anaesthetic to a patient.
2. Description of the Prior Art
An apparatus having a vaporizing unit which contains liquid anaesthetic, whereby a predetermined concentration of the anaesthetic in the respiratory gas is achieved by vaporizing a defined amount of the liquid anaesthetic in respiratory gas passed through the vaporizing unit is described in U.S. Pat. No. 3,794,027. The apparatus, an animal anaesthesia machine, has an entrance part via which the respiratory gas, or the gases that form the respiratory gas, is supplied to the apparatus and an inlet tube, which conducts the respiratory gas and the vaporized anaesthetic to the patient. The apparatus includes a canister to which a source of gas is connected via a vaporizer unit. The inlet tube conducts respiratory gas to the patient and an outlet tube conducts the respiratory gas from the patient via a common Y-piece. A second vaporizer unit is connected to the inlet tube. The two vaporizer units may be used separately or together and may contain the same or different anesthetics. The first vaporizer unit has a valve, which can admit the gas from the gas source through the vaporizer unit and divert the gas around the vaporizer unit. When the gas is led through the vaporizer unit, it will bubble through the liquid anaesthetic and vaporize it. The gas containing the anaesthetic is then conducted to the canister and from there to the patient. The second vaporizer unit has two hollow sections which communicate via two apertures. The first hollow section forms a part of the inlet line. A substantially triangular vane can be placed in two positions, one of which permits the gas only to pass through the first hollow section. When the vane is brought into its second position, the gas will be led into the second hollow section and pass through a wick which separates the second hollow section into two halves. The wick is partially placed in the liquid anaesthetic which has been absorbed by the wick. The gas containing anaesthetic then passes through the second aperture and out into the inlet line.
Control of the vaporization of the anaesthetic is, for both vaporizer units, performed manually by activating a switch system (valve and vane) for a defined time when vaporization shall take place. The concentration of anaesthetic depends mainly on two factors: the gas flow through the vaporizer unit, which will determine the amount of anaesthetic that is vaporized for each time unit, and the total time during which the gas is led through the vaporizer unit, which will determine the total amount of vaporized anaesthetic and thereby also the concentration. This results in a variation of the concentration during the time anaesthetic is supplied to the patient. In order to avoid concentrations that are too high or too low, an empirically established scheme for switching the vaporizer unit on and off should be followed. With this system losses of anaesthetic from the system, through leakage and such, cannot be taken into account.
Another known apparatus, described in U.S. Pat. No. 4,770,168, has a chamber with liquid anaesthetic and a positive displacement pump. Via a valve, gas can be diverted from a pipe to the chamber in which the gas becomes saturated with anaesthetic. The flow of gas from the chamber back to the pipe is controlled by the pump. By controlling the motor that drives the pump via a feedback system, depending on a total flow in the pipe or the anaesthetic concentration in the pipe, the concentration of anaesthetic may be maintained with higher accuracy than the previously described apparatus. The chamber may also include a thermistor. The motor may then also be controlled depending on the temperature in the anaesthetic chamber.
The pump in this known apparatus limits the useful range of the flow of anaesthetic saturated gas out into the pipe. This causes problems if a total flow is initially low, as is the case when small children or small animals are to be anaesthetized. Similar problems occur if the total flow is initially high. Furthermore, the required concentration level of anaesthetic may vary depending on the anaesthetic used and on the individual patient.